Thermocouples are often used in process engineering for temperature measurement. Their mode of operation is based on the thermoelectric effect; i.e. current flows in a circuit if its conductors are composed of two different metals or alloys and the contact points (solder joints) of the different metals or alloys are held at different temperatures. With such a circuit, it is possible to measure a voltage which is proportional to the difference in temperature of the two solder joints. In practice, one solder joint is kept at constant temperature and the other joint is the sensor at the point of measurement.
Suitable thermocouples and the basic values of the associated thermoelectric voltages have been standardized. Such thermocouples are particularly useful at very high temperatures and where the measurements are to be made at inaccessible points. In addition, by a suitable choice of thermocouples, it is possible to solve individual measurement problems and to cover fixed temperature ranges. For best results, it is desirable that the thermoelectric voltage per degree of temperature difference be as high and as a linear as possible.
To prevent thermocouples being damaged, altered, or destroyed by corrosive media in reaction spaces, they are protected by metal or ceramic tubes. In this case, they are called sheathed thermocouples. The thermocouple pair is mutually insulated by small ceramic tubes and inserted into a tube closed at one end. The outer tube provides protection from mechanical stresses; inner protection tubes are particularly useful for preventing the inward diffusion of gases which can affect the thermoelectric properties of the metal pair. The aforementioned resistance to mechanical and chemical influences represents merely a basic requirement which must be met by thermocouples used in industrial practice.
However, in special cases, such temperature-measurement instruments must meet additional requirements. An example of such a special case is a brick-lined high-temperature reactor which is used for the production of synthesis gas by gasification of fossil fuels such as oil or bituminous coal. In these reactors, the brick lining suffers gradual wear. To prevent destruction of the steel shell of the reactor by overheating, the wall thickness of the brick-lining must not fall below a certain minimum. In those gasification plants which, depending on the feed material, are operated at temperatures from about 1000.degree. C. to about 1700.degree. C., very accurate temperature measurement is necessary because of the high sensitivity of the brick-lining even where the permissible temperature is slightly exceeded. Specifically, the maximum margin of error must not exceed about 10.degree. C.
For measuring temperatures in this range, platinum-rhodium-platinum thermocouples are preferable. They are usually sheathed with a tube of alumina and are surrounded by an outer protective tube, usually of a ceramic material such as alumina.
In gasification plants, platinum-rhodium-platinum thermocouples are subject to damage by the reactor atmosphere. Due to the high pressure of up to about 10 MPa, it is unavoidable that the reaction gases, in particular hydrogen, carbon monoxide and gaseous sulfur compounds, reach the thermocouple. Hydrogen leads to grain boundary growth of the thermocouple material, the sulfur compounds cause embrittlement thereof, and both causes lead to destruction of the thermocouple, or at least to a falsification of the temperature measured thereby.
In the gasification of bituminous coal, in which slag is formed, slag can additionally penetrate the protective tubes and impair the strength of the alumina by lowering its melting point. This considerably facilitates access of hydrogen and gaseous sulfur compounds to the thermocouple. The same problems arise in the use of protective tubes which are composed of refractory metals or other ceramic materials.